Pain perception, or nociception, is mediated by the peripheral terminals of a group of specialized sensory neurons, termed “nociceptors.” A wide variety of physical and chemical stimuli induce activation of such neurons in mammals, leading to recognition of a potentially harmful stimulus. Inappropriate or excessive activation of nociceptors, however, can result in debilitating acute or chronic pain.
Neuropathic pain, which typically results from damage to the nervous system, involves pain signal transmission in the absence of stimulus, pain from a normally innocuous stimulus (allodynia) and increased pain from a normally painful stimulus (hyperalgesia). In most instances, neuropathic pain is thought to occur because of sensitization in the peripheral and central nervous systems following initial damage to the peripheral nervous system (e.g., via direct injury or systemic disease). Neuropathic pain is typically burning, shooting and unrelenting in its intensity and can sometimes be more debilitating than the initial injury or disease process that induced it.
Existing treatments for neuropathic pain are generally suboptimal. Opiates, such as morphine, are potent analgesics, but their usefulness is limited because of adverse side effects, such as physical addictiveness and withdrawal properties, as well as respiratory depression, mood changes, and decreased intestinal motility with concomitant constipation, nausea, vomiting, and alterations in the endocrine and autonomic nervous systems. In addition, neuropathic pain is frequently non-responsive or only partially responsive to conventional opioid analgesic regimens, or to treatment with other drugs, such as gabapentin. Treatments employing the N-methyl-D-aspartate antagonist ketamine or the alpha(2)-adrenergic agonist clonidine can reduce acute or chronic pain, and permit a reduction in opioid consumption, but these agents are often poorly tolerated due to side effects.
Another common condition for which existing therapies are insufficient or problematic is inflammation. Transient inflammation is a beneficial mechanism that protects mammals from invading pathogens. Uncontrolled inflammation, however, causes tissue damage and pain and is the underlying cause of many illnesses, including asthma, as well as other allergic, infectious, autoimmune, degenerative, and idiopathic diseases. Existing treatments often exhibit low, delayed or only temporary efficacy, undesirable side-effects and/or a lack of selectivity. There is a continuing need for new drugs that overcome one or more of the shortcomings of drugs currently used for immunosuppression or in the treatment or prevention of inflammatory disorders, including allergic disorders, autoimmune disorders, fibrogenic disorders, and neurodegenerative diseases, such as multiple sclerosis, amyotrophic lateral sclerosis, Alzheimer's disease, and Huntington's disease.
P2X7 receptor (“P2X7”) is a ligand-gated ion channel that is activated by ATP and is present on a variety of cell types, including microglia in the central nervous system and cells involved in inflammation and immune system function. In particular, P2X7 is involved in activation of lymphocytes and monocyte/macrophages leading to the increased release of pro-inflammatory cytokines (e.g., TNFalpha and IL-1beta) from these cells. Recent studies indicate that inhibiting P2X7 activation in situations of inflammation (e.g., rheumatoid arthritis and other autoimmune diseases, osteoarthritis, uveitis, asthma, chronic obstructive pulmonary disease and inflammatory bowel disease) or interstitial fibrosis results in a therapeutic effect. These and other studies indicate that P2X7 antagonists may find use in the treatment and prophylaxis of pain, including acute, chronic and neuropathic pain, as well as a variety of other conditions including osteoarthritis, rheumatoid arthritis, arthrosclerosis, inflammatory bowel disease, Alzheimer's disease, traumatic brain injury, asthma, chronic obstructive pulmonary disease, and fibrosis of internal organs (e.g., interstitial fibrosis).
Small molecule P2X7 antagonists are desirable for such therapies. The present invention fulfills this need, and provides further related advantages.